The invention concerns a bipolar battery and a method for making a partition wall to be used in such a battery.
Monopolar batteries are recognized by one or more electrodes of the same polarity being connected in parallel and combined within the same cell unit in one corner or at one side of each electrode and by the current via a metal connector being conducted through the cell and over to the next adjacent cell of the same construction. The electrodes comprises active materials pasted on grids of lead. The monopolar battery, is thus characterized, among other things, by a large amount of conducting material.
In bipolar batteries, the electrodes of opposite polarity are united via an electroconductive partition wall having the positive active material on one side and the negative active material on the other side of the same wall. Between the positive and the negative active material in each cell, there is a separator, consisting of a porous, acid resistant material, that separates the negative and positive electrodes and also contains elctrolyte. The electrodes and separators are stacked together to comprise a pile with current connectors in both ends. It is important that the pile is so designed that no stray currents can pass from one electrode in one cell to the electrodes of opposite polarity in another cell. Accordingly, there must not be any eletrolyte connection between the cells. The advantage of the bipolar design is thus that the current transport occurs from one cell to the adjacent cell through the partition wall and not from the active material to current collecting grids and via a lug, a post or a connector to the adjacent post, lug grid and finally to the active material in next cell. The weight savings may for this reason be considerable and the inner resistance substantially lower.
In bipolar lead acid cells so far known, the positive active material (PAM), which is porous lead dioxide, must be in direct electric contact with the conducting partition wall (e.g. made from metallic lead), which also contacts the negative active material (NAM), which is porous lead. The lead wall will corrode during repeated charging and discharging if no special measures are taken. The elelctrolyte in one cell may then come into contact with the electrolyte in the adjacent cell and shortcircuiting (stray currents) appears. In all lead acid batteries volume changes of the active materials appear during charging and discharging. In the discharge process, lead sulphate (PbSO.sub.4) is formed from both lead (NAM) and PbO.sub.2 (PAM), and assumes a substantially larger volume than the compounds from which it was formed. Corrosion of the positive side of the lead wall also brings about changes in the volume; non porous Pb is transformed to PbO.sub.2. If the conducting wall does not have enough mechanical strength to resist the forces from the expansion of the lead and the lead dioxide during discharge or from the increase in volume due to the corrosion of lead, the wall will bulge and crack with break through and shortcircuiting as the result. It will also be difficult to retain the tightness with respect to the other cells in the pile.
U.S. Pat. No. 4,297,421 concerns an electrode for the use in e.g. bipolar batteries, which is intended to solve the above problems. To this end the electrode consists of a composite of a anodically passivatable metal, e.g. titanium, infiltrated by a metal capable of forming an electroconductive oxide, e.g. lead. This electrode, however, suffers from a number of disadvantages. The passivation on the positive side of the matrix metal reduces the desired high conductivity of the electrode. The matrix metal (Titanium) will dissolve on the negative side, thus severly reducing the working life of the electrode. Further the matrix metals intended for use are very expensive as is the manufacturing of the electrode due to the difficulty in working with these metals.
If the wall was to be manufactured from a porous non-dimentional stable material, as e.g. a thermoplastic material, the pores would grow in volume when they are exposed to the increasing volumes caused by the transformation of lead to PbO.sub.2 and PbSO.sub.4. These compounds would then expand more or less without control and form porous layers giving the electrolyte and current the possibility to penetrate further into the pores. This would result in fast corrosion and shortciruits, why these materials are not suited in partition walls.